In many areas of the United States, direct impingement solar collectors provide a convenient way to harness solar energy. Typically, such collectors employ a fluid circulating in a large flat, rectangular core of the collector, enabling solar radiation impinging on the flat surface to be transferred directly through conduction to the circulating fluid. Obviously, with the appropriate plumbing system, the heat energy absorbed in the fluid can be put to useful work, such as warming a swimming pool, heating a home or for processes requiring heat energy. The basic unit in such systems is the collector. It generally consists of an absorber, which can be fabricated of glass, metal or plastic. Generally, the absorbers are constructed to provide a larger planar absorbing surface which is panel-like and is referred to as the "core". This surface is designed to be directly exposed in the most advantageous orientation to receive solar radiation which is normal to the radiation.
Typically, in such panels or cores, a large number of passages or channels are provided, on or directly beneath the exposed absorbing surface, which of course increases in efficiency as its emissivity approaches unity. Since normally the channels underlying the absorption surface are axially aligned, a header is provided at each end of the core to act as a manifold for distributing fluids circulating in the core structure. One header acts as the input header and distributes fluid to the plurality of channels in the core. Under the influence of pressure, this fluid feed migrates through the core to the opposite header, where it is removed after having been warmed by conduction as a result of its transit through the core. Generally the combination of the headers and the intermediate core which extends therebetween is referred to as the absorber.
While the above description covers the more common type of collectors, there are other devices of a more costly and esoteric design. In reference to the units described, glass absorbers are currently prohibitively expensive. As a result, based on the current economic conditions, glass absorbers will normally result in a net loss, considering the investment costs and energy savings during their useful life. Also, they are fragile and difficult to install. Like glass absorbers, metal absorbers are quite expensive, and both glass and metal absorbers are particularly susceptible to freezing temperatures if water is employed. Also, metal absorbers tend to have corrosion problems as well as being electrolytically active in certain situations. This makes them unattractive for heating corrosive fluids. As a result, plastic absorbers, which are far less expensive, represent a large volume of such units sold today. They are particularly suitable since they are non-corrosive, resist scaling, and can stand mild freezing temperatures without damage. However, these plastic collectors, while having a number of desirable attributes, have relatively high coefficients of expansion, which must be taken into consideration in any proper installation.
Examples of plastic absorbers used as collectors are shown in U.S. Pat. No. 3,239,000 issued to Meagher. As can be seen from a perusal of these patents, the absorbers are typically a panel or core comprising a relatively thin sandwich section of plastic in a rectangular configuration, having a plurality of longitudinal, parallel channels disposed between the top and bottom surfaces. These channels at their ends are connected between two headers or manifolds which serve to distribute and control the fluid flow through the core. The fluid, heated during the transit through the core, is usually disseminated in a plumbing system where its absorbed heat energy can be utilized.
With proper construction and compounding of the plastic, the service life of plastic absorber cores or panels can be greatly enhanced. In particular, the polyolefins, such as polypropylene and polyethylenes, and/or mixtures thereof, are particularly suitable when combined with the suitable antioxidants and colorants, to protect these plastic panels from degradation due to heat and ultra-violet impingement on the surfaces. Carbon black is often added to the plastic, which provides some ultra-violet protection and also colors the core toward black, improving its emissivity factor. This tends to increase the operating temperatures and thus the efficiency of the core. As the operating temperature increases, the plastics become more pliable and flexible. Thus, an absorber constructed of plastic tends to be subjected to a great deal of stress as temperatures elevate and the core requires more support. This stress is generated, in part, by the high coefficient of expansion of the plastics and a necessity to tether the absorber unit on a surface, such as a roof or the like. Of course, the increased flexibility of the absorber when combined with environmental forces, such as the wind, can lead to a situation where the absorber panel is severely stressed or damaged. Sometimes the absorber panel can be literally torn from the roof or other supporting surface.
In many installations, multiple absorbers are employed in a nested arrangement, i.e., side-by-side mounting upon a surface such as a roof. Generally, such a group of absorbers can be referred to as "nested absorbers" and of course, provides the necessary surface area to collect the required amount of solar energy. The absorbers are generally 4 foot.times.8 foot panels nested together on a roof, and it can be appreciated that a number of mounting problems occur because the absorbers must be secured sufficiently to hold them on the roof. When the absorbers are secured, expansion and contraction of such nested absorbers must be taken into account, which can be as great as 6 to 8 inches in a 24-hour period. Obviously, such movement can develop serious stresses in the absorbers if they are improperly mounted on a roof or other exterior surface. Typically, in such installations, leakage develops as a result of absorber movement, and damage to the roof or other supporting surface is commonplace, due to the stress placed on the moorings securing the units.
Also, absorbers which have irregular exterior surfaces tend to collect debris which can accumulate in depressions, crevices or cups in the associated absorber. As a result, leaves and other debris may collect there and tend to accelerate the degradation of the plastics used in such absorbers. Accumulation of debris should be avoided, if possible, particularly in areas on a plastic surface which also can accumulate water, as it lowers efficiency.
Another problem with plastic collectors is abrasion of the absorber caused by the expansion and contraction which occurs daily. Further, when these collectors become heated, they tend to conform to the configuration of the underlying roof. As a result, the abrasion problem is accentuated due to a warped condition of the unit when it cools. Further, problems develop under the absorber when it rests against the surface of a roof. First, dry rot is likely to occur. Secondly, the moisture between the absorber and the roof increases thermal conductivity and results in heat loss from the absorber, reducing its efficiency.
Warpage of a plastic absorber in some cases may be extreme and may contribute to a substantial reduction of its service life due to the stresses developed in the absorber panel.
Beyond absorber service life and thermal efficiency considerations, an additional consideration is the techniques for mechanically securing the absorber to the roof. Due to its coefficient of expansion, the absorber must be allowed to move. Also, if it is supported away from the roof to avoid warpage and dry rot, wind can get under the flexible collector and distort it significantly. Under some situations, a flexible collector may be converted into an airfoil, placing significant stresses on mechanical hold-down structures.
All of the above problems can be overcome or significantly reduced by placing a plastic absorber in a metal frame system forming a new combination structure according to this invention, which, in addition to solving the mentioned problems, may increase the efficiency of the collector by 20% to 50% when glazing is added. The increased efficiency is due to the hothouse effect which raises the temperature of the air in immediate contact with the absorber. Normally, this temperature increase would be undesirable because of the above-described problems, however, due to the unique support system provided according to this invention, no difficulties are experienced at such elevated temperature operation.